CD8+ T lymphocytes (CTL) are believed to be a crucial arm of immunity in HIV-1 infection. Data from studies of immunopathogenesis suggest that CTL exert an important antiviral effect that suppresses viral replication in acute and chronic infection, and potentially contributes to protection of some persons from infection with HIV-1. Thus, many vaccine efforts have placed heavy emphasis on provoking CTL responses. However, the factors which influence CTL antiviral activity in vivo, are poorly understood. CTL fail to control viral replication and prevent eventual disease progression in most infected persons. It appears that HIV-1 can mutate to escape antiviral CTL, and that the Nef protein may render infected cells resistant to the activity of CTL. This research plan proposes to examine whether CTL that target the Nef protein may be functionally more effective than those that target other proteins. This is based on the observations that Nef is an early protein that downregulates MHC class I molecules, and Nef is highly conserved in vivo despite being dispensable in vitro. We intend to utilize assays that measure the actual antiviral effect of CTL exposed to HIV-1-infected cells, which are a more physiologic measure of antigen expression and CTL function than assays in common use (that utilize recombinant or synthetic antigens and measure indirect correlates of antiviral activity such as interferon gamma release). Specifically, we plan: 1. To compare the antiviral efficiencies of CTL recognizing epitopes in Nef versus epitopes in other structural and regulatory proteins ofHIV-1; 2. To evaluate the functions of Nefmutants selected by Nef-specific CTL (in vitro); 3. To evaluate potential in vivo selective pressures for conservation of Nef.